- Dropbox files at rest are encrypted using 256-bit Advanced Encryption Standard (AES).
- Dropbox uses Secure Sockets Layer (SSL)/Transport Layer Security (TLS) to protect data in transit between Dropbox apps and our servers; it's designed to create a secure tunnel protected by 128-bit or higher Advanced Encryption Standard (AES) encryption.
Source: https://www.dropbox.com/help/27